This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-085816, filed Mar. 23, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a printed circuit board that has footprints to which a surface mount device may be soldered, to a circuit module that comprises a printed circuit board to which surface mount devices are soldered, and to a method of manufacturing a printed circuit board that has footprints.
2. Description of the Related Art
A surface mount device, such as a bare chip, a SOP (Small Outline Package) or a QFP (Quad Flat Package), is secured to the mounting region of a printed circuit board by reflow soldering. The reflow soldering is performed in the following sequence of steps.
First, a printed circuit board is prepared, which has a mounting region and a plurality of footprints formed in the mounting region. Next, solder paste is applied onto the footprints, forming solder layers that cover the footprints. Then, the surface mount device is mounted at the prescribed position in the mounting region, by using a parts mounter. The terminals of the surface mount device therefore contact the solder layers. Thereafter, the printed circuit board is placed in a reflow oven and heated therein. As the board is heated, the solder layers melt and spread over the footprints. Finally, the printed circuit board is cooled, hardening the solder layers. As a result, the terminals are connected to the footprints, both electrically and mechanically. The surface mount device is thereby secured to the printed circuit board.
The footprints provided on the conventional printed circuit board are rectangular strips, each having a size proportional to the terminal it holds. Being rectangular, each footprint has an area greater than is necessary. Inevitably, the amount of solder applied to each footprint must be larger than necessary. Consequently, the solder flows in a large amount on each footprint, from the center part of the footprint toward the corners thereof, when the printed circuit board is heated in the reflow oven.
As the molten solder so flows, the surface mount device may move from the desired position or may rotate, even if the device has been placed at the very desired position in the mounting region of the printed circuit board. To state it more precisely, the molten solder is likely to flow toward the corners of the footprint, because the right-angled corners lie outside the terminal it holds. As the molten solder so flows, the surface mount device may rotate with respect to the footprints. If the printed circuit board is cooled in this condition, the surface mount device is secured to the board, rotated from the desired position, possibly interfering with the other circuit devices provided on the board or causing short-circuiting on the board.
After the soldering, the printed circuit board is inspected. If any surface mount device is found to have rotated in excess, it must be removed from the mounting region of the board and soldered again at the desired position in the mounting region. This repairing requires much time and labor, reducing the efficiency of mounting the device on the printed circuit board.
Jpn. Pat. Appln. KOKAI Publication No. 2000-261131 discloses a modified method of soldering electronic devices of surface mount type to a printed circuit board. The method uses footprints, each comprising two parts that are spaced apart from each other. Therefore, after the terminals of an electronic device are soldered to the footprints provided on the board, each terminal straddles the two parts of one footprint.
The soldering method disclosed in Publication No. 2000-261131 is devised mitigate the stress generated at the junction between footprints and terminals if the terminals are soldered to the footprints, by using lead-free solder, i.e., solder that has high rigidity. Publication No. 2000-261131 teaches no techniques of preventing the surface mount device from moving from the desired position.
Both parts of every footprint disclosed in Publication No. 2000-261131 are rectangular, each having four right-angled-corners. The footprints are not different at all, in shape, from those provided on the conventional printed circuit. The molten solder tends to flow toward the corners of either part of each footprint as the printed circuit board is heated during the reflow soldering. Hence, it is impossible to prevent the surface mount device from moving from the desired position in the process of soldering the device to the printed circuit board.
An object of the present invention is to provide a printed circuit board on which a surface mount device can be prevented from moving while being soldered to the board and can therefore be arranged at a desired position and to provide a method of manufacturing a printed circuit board of this type.
Another object of the invention is to provide a circuit module that has a printed circuit board of the type described above.
To attain the objected mentioned first, a printed circuit board according to a first aspect of the invention comprises: a substrate having a mounting surface on which the surface mount device is to be mounted; and at least one footprint arranged on the mounting surface and positioned to align with the terminal of the surface mount device. The footprint comprises a plurality of patterned parts and a plurality of solder layers formed on the patterned parts, respectively. Each of the patterned parts is so shaped to limit a region in which molten solder flows when the solder layer is melted.
To achieve the first-mentioned object, a printed circuit board according to a second aspect of the invention comprises: a substrate having a mounting surface on which a surface mount device having a plurality of leads arranged at intervals is to be mounted; and a plurality of footprints arranged, side by side, on the mounting surface of the substrate and positioned to align with the leads, respectively. Each of the footprints comprises a plurality of patterned parts and a plurality of solder layers formed on the patterned parts, respectively. The patterned parts are arranged in a column, spaced apart in a lengthwise direction of the lead and so shaped to limit a region in which molten solder flows when the solder layer is melted.
To attain the second-mentioned object, a circuit module according to a third aspect of the invention comprises: a surface mount device having at least one terminal; a printed circuit board having a mounting surface on which the surface mount device is soldered; and at least one footprint arranged on the mounting surface and positioned to align with the terminal of the surface mount device. The footprint comprises a plurality of patterned parts and a plurality of solder layers formed on the patterned parts, respectively. Each of the patterned parts is so shaped to limit a region in which molten solder flows when the solder layer is melted.
A method of manufacturing a printed circuit board, according to a fourth aspect of the invention, comprises: forming at least one footprint on a mounting surface of a substrate and in alignment with a terminal of a surface mount device, the footprint comprising a plurality of patterned parts which have a shape selected from the group consisting of a circular shape, a rectangular shape having rounded corners, and a polygonal shape having at least five corners; coating the patterned parts with solder paste through a screen mask laid on the mounting surface of the substrate, thereby covering the patterned parts with solder layers; and applying solder resist to the mounting surface of the substrate, except regions in which the patterned parts are provided.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.